The term “wettability” refers to the adhesion tension, or the tendency of a particular fluid to spread on or adhere to a solid surface in the presence of another immiscible fluid. The term “formation wettability” refers to the ability of a rock surface to preferentially contact a particular fluid. It is a function of the solid-liquid-liquid interfacial tension and determines which fluid (oil or water) will preferentially wet (adhere to) the solid surface of the rock. If the adhesion tension is large, the denser phase liquid will readily spread out and tend to coat the surface of the rock. If the adhesion tension is small, the denser fluid will only be weakly attracted to the surface. Formation wettability can affect the formation's properties such as, but not limited to, relative permeability, capillary pressure, fluid location, fluid flow, and residual oil distribution. By changing the rock character to be more water-wet, oil will flow more freely rather than being bound to the rock surface.
Formations tend to be fractured due to geologic and tectonic activity. Thus the rock matrix, within which the oil is contained, is often surrounded by fractures which are much more conductive to flow than the rock matrix itself. Under fractured rock conditions, an imbibition process, a process by which injected brine is spontaneously sucked into the rock and ejects oil simultaneously, is the main mechanism of recovery of oil. When a formation is not fractured, the main mechanism may simply be a physical displacement process also known as viscous displacement. In either case, wettability of the rock must be converted to water-wet conditions partially or fully so that the oil will flow more freely rather than being bound to the rock surface.
Under oil-wet conditions, the oil tends to be retained by the rock and hence results in low mobility of the oil. The low mobility of the oil in turn results in poor recovery of the oil from the formation. As an example, carbonate rocks tend to be more oil-wet in many cases and mixed-wet in some cases. It is desirable to engineer wettability and optimize wettability alteration in order to obtain improved recovery from rock systems.
Therefore, there exists a need to “optimize” or engineer wettability characteristics of formations to improve oil production methods and strategy.